The present invention relates to a process for separating a C4-hydrocarbon mixture essentially containing 1,3-butadiene, butenes, butanes and other C4-hydrocarbons into at least 4 fractions,
a) the fraction (a) essentially comprising 1,3-butadiene,
b) the fraction (b) essentially comprising butenes,
c) the fraction (c) essentially comprising butanes and
d) one or more fractions (d) essentially comprising [lacuna] the other C4-hydrocarbons,
xe2x80x83by extractive distillation by means of N-methyl-2-pyrrolidinone or an aqueous solution of N-methyl-2-pyrrolidinone (NMP),
xe2x80x83wherein
1. the gaseous C4-hydrocarbon mixture is first brought into contact with NMP in an extraction zone (I), the 1,3-butadiene and the other C4-hydrocarbons being essentially completely absorbed by the NMP but the butenes and butanes remaining essentially in the gas phase;
2. the unabsorbed butenes and butanes (gas stream bc) and the extraction solution formed in step 1 (extraction solution ad) are removed from the extraction zone (I);
3. the extraction solution (ad) is transferred to a desorption zone (I) at a lower pressure and/or higher temperature than the extraction zone (I) and 1,3-butadiene is desorbed from the extraction solution (ad), the main part of the other C4-hydrocarbons remaining in the liquid phase;
4. the extraction solution formed in stage 3 (extraction solution d) and the desorbed 1,3-butadiene (fraction a) are removed separately from the desorption zone (I) and, if required, a part of the fraction (a) is returned to the extraction zone I;
5. the extraction solution (d) is transferred to a second desorption zone (II) at a lower pressure and/or higher temperature than the desorption zone (I) and having a pressure and/or temperature gradient, and the other C4-hydrocarbons and the 1,3-butadiene still remaining therein are fractionally desorbed from the extraction solution (d) as at least two separate fractions (d), with the content of the other C4-hydrocarbons being at least 10 times higher in at least one of the fractions (fractions d) than in the extraction solution (d), based on the content of all C4-hydrocarbons, and the content of the other C4-hydrocarbons being correspondingly lower in at least one of the fractions (fractions dR) than in the fractions (d), based on the content of all C4-hydrocarbons,
6. the NMP, formed in the desorption zone (II) and essentially free of C4-hydrocarbons, and the fractions (d) and (dR) are removed separately from the desorption zone II, and one or more fractions (dR) are returned to the desorption zone (I),
7. the gas stream (bc) is first brought into contact with the NMP formed in step 6 in an extraction zone (II), the butenes being essentially completely absorbed by the NMP but the butanes remaining essentially in the gas phase;
8. the unabsorbed butanes (fraction c) and the extraction solution formed in step 1 (extraction solution b) are removed from the extraction zone (II);
9. the extraction solution (b) is transferred to a desorption zone (III) at a lower pressure and/or higher temperature than the extraction zone (II) and the butenes are desorbed from the extraction solution (b);
10. the NMP, formed in step 9 and essentially free of C4-hydrocarbons, and the desorbed butenes (fraction b) are removed from the desorption zone (III);
11. the NMP formed in step 9 is recycled to one of the extraction zones.
This process is shown schematically in FIG. 1.
A process for separating 1,3-butadiene from a C4-hydrocarbon mixture is disclosed, for example, in DE-A-2724365. Briefly, a butane/butene mixed fraction, a 1,3-butadiene fraction and a fraction which contains the other C4-hydrocarbons are obtained in this process from a C4-hydrocarbon mixture which contains butanes, butenes, 1,3-butadiene and other C4-hydrocarbons, by extractive distillation with NMP as absorbent over various absorption and desorption stages. In the entire process, the NMP required passes through a closed circulation. NMP which no longer contains any C4-hydrocarbons (unladen NMP) is first laden with the C4-hydrocarbon mixture at the beginning of a cycle, passes through the various absorption and desorption stages until, at the end of a cycle, unladen NMP is provided by completely desorbing the C4-hydrocarbons. The process is distinguished by the fact that the individual stages are particularly advantageously coupled via indirect heat exchange processes.
The separation of 1,3-butadiene and 2-butenes into separate fractions and the separation of 1,3-butadiene and acetylenes into separate fractions from C4-hydrocarbon mixtures have been described by V. A. Gorshkov et al. in the publication The Soviet Chemical Industry, No. 11, November 1971.
EP-A-141356 likewise relates to the separation of a 1,3-butadiene fraction from a C4-hydrocarbon mixture by means of extractive distillation using NMP. The use of columns in which absorption and desorption zone are integrated in a single column in each case make this process particularly economical.
EP-A-5788 discloses a process for separating a 1,3-butadiene fraction and a butyne fraction from a C4-hydrocarbon mixture by means of extractive distillation using NMP.
EP-A-9630 relates to a process for separately removing styrene and 1,3-butadiene from a mixture which otherwise contains C4-hydrocarbons, the styrene first being separated from the mixture by distillation and 1,3-butadiene being separated from the remaining mixture by means of extractive distillation.
U.S. Pat. No. 5,242,550 discloses the separation of a butene/butane mixture by means of extractive distillation using NMP as absorbent.
It is an object of the present invention to provide a process which permits the separation of a C4-hydrocarbon mixture into a butane fraction, butene fraction, 1,3-butadiene fraction and a fraction which contains the other C4-hydrocarbons in a particularly efficient and economical manner. In particular, the required quantities of energy and the capital costs should be particularly low in this process.
We have found that this object is achieved by the process described at the outset.
The process can be applied to C4-hydrocarbon mixtures which contain 1,3-butadiene, butenes, butanes and other C4-hydrocarbons plus very small amounts of C3- and C5-hydrocarbon impurities.
Such C4-hydrocarbon mixtures are obtained, for example, as C4 fractions in the production of ethylene and/or propylene by thermal cleavage of a petroleum fraction, for example of liquefied petroleum gas (LPG), naphtha, gas oil or the like as hydrocarbon fraction. Furthermore, such C4 fractions are obtained in the catalytic dehydrogenation of n-butane and/or n-butene. The C4 fractions obtain [sic], as a rule, butanes, n-butene, isobutene, 1,2 butadiene, vinylacetylene, ethylacetylene and 1,2-butadiene [sic] and may contain small amounts of C5-hydrocarbons, the 1,3-butadiene content being in general from 10 to 80, preferably from 20 to 70, in particular from 30 to 60, percent by weight while the content of vinylacetylenes, ethylacetylene and 1,2-butadiene (referred to below as other hydrocarbons) together in the C4 fractions generally does not exceed 5 percent by weight.
The novel process can advantageously be employed in particular to those C4-hydrocarbon mixtures which contain
from 10 to 80% by weight of 1,3-butadiene;
from 10 to 60% by weight of butenes;
from 5 to 40% by weight of butanes;
from 0.1 to 5% by weight of other C4-hydrocarbons and
from 0 to at most 5% by weight of C3- and C5-hydrocarbons.
The n-methyl-2-pyrrolidinone or its aqueous solution employed as selective solvent (N-methyl-2-pyrrolidinone and its aqueous solution abbreviated to xe2x80x9cNMPxe2x80x9d for short hereinafter) is generally a conventional industrial product which may contain up to 15% by weight of water.
The extraction zones are preferably in the form of columns through which the gas streams are passed countercurrently to the NMP.
In step 1, the C4-hydrocarbon mixture to be separated is first fed in gaseous form with NMP into an extraction zone (I) and brought into contact with one another there, the 1,3-butadiene and the other C4-hydrocarbons being essentially completely absorbed by the NMP but the butenes and butanes remaining essentially in the gas phase. In the NMP and C4-hydrocarbon mixture fed in, the ratio of NMP to C4-hydrocarbon mixture is from 5:1 to 20:1 in the extraction zone (I).
The generally known extraction methods are suitable for this extraction step.
From the extraction zone (I), in general a gas stream which [lacuna] in particular unabsorbed butanes and butenes and, if C3- and C5-hydrocarbons are present as impurity in the C4 mixture, also propane, propene and propadiene plus traces of C5-hydrocarbons (gas stream bc) is removed at the top of column and the extraction solution (extraction solution ad) is removed from the bottom of the column.
The extraction solution (ad) contains in general only from 0 to 2% by weight of butenes and butanes, plus, if present, propyne and/or almost the total amount of C5-hydrocarbons.
The gas stream (bc) contains, in addition to the butenes and butanes, in general only from 0 to 1% by weight of the 1,3-butadiene originally present in the C4-hydrocarbon mixture and of the other C4-hydrocarbons.
The extraction zone (I) is generally in the form of a scrubbing column with plates, dumped packings or structured packings as internals. These preferably have from 40 to 80 theoretical plates. The column pressure depends on the temperature of the cooling medium (well water, river water, sea water, refrigerants such as liquid propylene, liquid ammonia or brine). It is between 2 and 6 bar, preferably 4.5 bar. The temperature profile in the extraction zone is determined by the temperature of the NMP. It is advantageous to lower the temperature profile by partial condensation of the fraction (bc) because the separation efficiency is improved at lower temperature. A typical value for the condensation is 20%. This results in a temperature of from 40 to 60xc2x0 C. at the top of the column.
For the desorption of the 1,3-butadiene from the extraction solution (ad), the latter is transferred to a desorption zone (I) at a lower pressure and/or higher temperature than the extraction zone (I) and 1,3-butadiene (1,3-butadiene fraction a) is desorbed from the extraction solution (ad), the main part of the other C4-hydrocarbons, propyne and C5-hydrocarbons remaining in the liquid phase.
Preferably, the pressure in the desorption zone (I) is the same as that in the extraction zone (I) and the temperature is from 20 to 25xc2x0 C. higher than in the extraction zone (I).
The 1,3-butadiene fraction (a) removed from the desorption zone (I) usually has a purity of from 95 to 99% by weight.
The extraction solution (d) formed by desorption of 1,3-butadiene in the desorption zone (I) is then removed from the desorption zone (I) and transferred to a second desorption zone (II) at a lower pressure and/or higher temperature than the desorption zone (I). During transfer of the extraction solution (d) from desorption zone (I) to (II) it advantageously passes through a heat exchanger zone in which a part of the hydrocarbons in the extraction solution (d) evaporates, and this gas stream is directly fed back into the bottom of desorption zone (I). Pressure and temperature are chosen so that virtually all C4-hydrocarbons still remaining in the NMP are desorbed; they are in general 1.5 bar and 150xc2x0 C.
In desorption zone II there is fractional desorption from the extraction solution (d) of 1,3-butadiene still present therein and of the other C4-hydrocarbons plus, where appropriate, [lacuna] and C5-hydrocarbons still present therein as at least two separate fractions (d), with the content of other C4-hydrocarbons being at least 10 times, in general from 10 to 100 times, preferably from 20 to 80 times, higher in at least one of the fractions (fraction d) than in the extraction solution (d), based on the content of all C4-hydrocarbons, and the content of the other C4-hydrocarbons being lower in at least one of the fractions (fractions dR) than in the fractions (d), based on the content of all C4-hydrocarbons. The hydrocarbons in the extraction solution (d) are preferably fractionated in the desorption zone (II) into a fraction (d) and a fraction (dR), where fraction (d) preferably comprises essentially at least 20% by weight, particularly preferably from 20 to 40% by weight, of other C4-hydrocarbons and otherwise butadiene, and fraction (dR) comprises essentially more than 80% by weight, particularly preferably from 85 to 95% by weight, of butadiene and otherwise other C4-hydrocarbons.
The NMP formed in the desorption zone (II) and essentially free of C4-hydrocarbons, and fractions (d) and (dR) are removed separately from the desorption zone II, and one or more of the fractions (dR) are returned to the desorption zone (I), e.g. to the bottom of the scrubbing column.
The pressure gradient in this case is preferably overcome by means of a compressor. The fraction (d) is normally treated countercurrently with water (condensate) in order to absorb most of the NMP present therein.
In general, the ratio by weight of the fractions returned to the desorption zone (I) to those removed from the system is from 20:1 to 80:1.
The desorption zone (II) consists in general of a main column with a side column. Both are designed as scrubbing columns. The main column generally contains packings because the low pressure drop thereof has particularly beneficial effects here. The main column should have from 10 to 15 theoretical plates. The side column generally has 10 practical plates. The pressure is generally from 1.5 to 1.6 bar; the temperature at the bottom of the main column is from 140 to 150xc2x0 C. and at the top thereof is from 80 to 100xc2x0 C. While the fractions (d) are removed as sidestream preferably at from 130 to 140xc2x0 C., the fractions (dR) are normally taken off overhead.
If a 1,3-butadiene fraction having a particularly high purity is desired, the following procedure is preferably adopted:
The 1,3-butadiene fraction (a) which is removed from the desorption zone (I) is divided into two part streams of fraction (a1) and (a2), and fraction (a1) is returned to the extraction zone I (is preferably passed to the bottom of the extraction column I) and fraction (a2) is again brought into contact, in an extraction zone (III), with NMP which was recovered from the desorption zone (II) or (III), a part of the fraction (a2) and the predominant part of other C4-hydrocarbons still contained as impurity in the fraction (a2) being absorbed by the NMP (extraction solution ax).
The unabsorbed part of the fraction (a2) (fraction a3) is removed separately from the extraction zone, and the extraction solution (ax) is returned to the extraction zone (I).
This variant is illustrated in FIG. 2.
The ratio by weight of NMP to 1,3-butadiene fraction (a) generally corresponds to from 1:3 to 1:7, depending on the composition of the initial C4 mixture and the specifications for fraction (a3).
The ratio of the material streams of fractions (a1) and (a2) is normally from 1:1 to 4:1.
The 1,3-butadiene fraction (a3) still contains impurities in particular in the form of C3- and C5-hydrocarbons and 1,2-butadiene. These impurities are in general subsequently removed in two conventional distillation columns.
Regarding the design of the extraction column and the parameters of pressure and temperature, the same applies in general terms to the extraction zone (III) as to the extraction zone (I). The ratio of NMP fed in to the crude butadiene fraction (a2) corresponds to from 1:3 to 1:7.
The unabsorbed 1,3-butadiene and the 1,3-butadiene taken off from the extraction zone (III) normally has a purity of more than 98% by weight.
The gas stream (bc), optionally with the addition of the external added stream (gas stream Zbc is first brought into contact, in an extraction zone (II), with the NMP recovered in the desorption zone (II), the butenes being essentially completely absorbed by the NMP but the butanes remaining essentially in the gas phase.
The extraction zone (II) is in general in the form of a scrubbing column with plates, dumped packings or structured packings as internals. These must have from 30 to 70 theoretical plates in order to achieve a sufficiently good separation effect. The pressure in the extraction zone (II) is chosen so that the gas stream (bc) is able to pass from the extraction zone (I) without further technical assistance into the extraction zone (II). It also depends on the cooling medium available for condensing the fraction (c). A typical value for the pressure is 4.0 bar, provided water is used for cooling.
The scrubbing column is advantageously equipped in the top of the column with a back-wash zone which comprises, for example, 4 theoretical plates. This back-wash zone serves for recovering the NMP present in the gas phase by means of back-flow of liquid hydrocarbon, for which purpose fraction (c) has previously been condensed. It is possible at the same time to influence thereby the temperature profile in the extraction zone (III). It also applies in this case, as already mentioned in extraction zone (I), that a lower temperature promotes the separation efficiency. Typical temperatures at the top of the column are between 35 and 45xc2x0 C.
The ratio by weight of NMP to gas stream (bc), including gas stream Zbc where appropriate, in the feed to extraction zone (II) is from 10:1 to 20:1, depending on the specifications for fractions (b) and (c) and the composition of the initial C4 mixture and of the added stream Zbc.
In the extraction zone (II), a gaseous butane fraction (fraction c) and an extraction solution (b) containing the butene fraction (fraction b) are formed. If the extractive distillation is carried out as described above, a fraction (b) which is contaminated with up to 5% by weight of butanes and a fraction (c) which is contaminated with up to 15% by weight of butenes are obtained.
The extraction solution (b) is transferred to a desorption zone (III) at a lower pressure and/or higher temperature than the extraction zone (II), the butenes being desorbed from the extraction solution (b). The desorption of the butenes and of any other C4-hydrocarbons contained therein as impurity can in principle be carried out similarly to the desorption of the other C4-hydrocarbons in the desorption zone (II).
The desorption zone (III) may be, for example, in the form of a scrubbing column which has from 5 to 15, preferably from 8 to 10, theoretical plates and a back-wash zone with, for example, 4 theoretical plates. This back-wash zone serves for recovering the NMP present in the gas phase by means of a back-flow of liquid hydrocarbon, for which purpose the fraction (b) has previously been condensed. It is advantageous to provide packing beds as internals. The pressure at the top of the column is generally 1.5 and [sic] 1.6 bar. The temperature in the bottom of the column is generally from 130 to 150xc2x0 C.
The NMP recovered in the desorption zone (III) is returned to the extraction zones (I), (II) and/or (III).
An additional advantage accrues when the NMP recovered in the desorption zone (III) is fed back only into the extraction zones (I) and (II), and the NMP recovered in the desorption zone (II) is essentially fed back into the extraction zone (III).
The advantage derives from the fact that the removal of butenes/butanes from a C4-hydrocarbon mixture using NMP takes place more easily than the separation of a mixture of butenes and butanes into two high-purity butene and butane fractions. Moreover a single solvent circulation is maintained.
In contrast to the extraction zone (III), which requires high-purity degassed NMP, the quality of the NMP for the extraction zones (I) and (II) does not need to be so high. This signifies a gain economically in that the degree of degassing of the NMP, and thus the consumption of external steam for desorbing hydrocarbons in the desorption zone (III), does not need to be so high. In contrast to the NMP from the desorption zone (II), where from 0 to 10 ppm by weight of C4-hydrocarbons are desired, it is perfectly possible for the NMP from the desorption zone (III) to have 1000 or more ppm by weight. This does not impair the purity of product fractions (b) and (c). On the other hand, however, a content of hydrocarbons reduces the boiling point of the solvent. Since the heat content of the NMP from the desorption zone (III) is utilized, however, for reasons of economy, it is not possible to continue reducing the boiling point by increasing the residual content of hydrocarbons indefinitely. The bottom temperatures of from 130 to 150xc2x0 C. indicated previously result for these reasons. At a bottom temperature of 138xc2x0 C., the resulting residual content of hydrocarbons is about 800 ppm by weight.
The novel process can be carried out particularly economically if the heat of the NMP which is obtained by boiling up the extraction solutions (b) and (d) is fed to the desorption zone (I), (II) and/or (III) by indirect heat exchange in a heat exchange zone, and the desorption is effected in these desorption zones by increasing the temperature in the desorption zone (I) relative to that in the extraction zone (I), and increasing the temperature in the desorption zone (II) relative to that in the desorption zone (I) and increasing the temperature in the desorption zone (III) relative to that in the extraction zone (II).
The separation of the fraction (a) (butadiene) from the C4-hydrocarbon mixture is preferably carried out as described in DE-A-2724365. This part of the process is particularly preferably carried out as described in FIG. 3.
According to this process variant, the following procedure is adopted:
The extractive distillation is carried out in more than one column, in general in two columns which together have more than 100 practical trays. When using two columns, the absorption stage situated above the point at which the C4-hydrocarbon mixture is fed into the extractive distillation zone is advantageously located in the first column and the concentration stage situated below the feed point of the hydrocarbon mixture is advantageously located in the second column, i.e. the feed point for the hydrocarbon mixture is at the top of the second column or preferably at the bottom of the first column. Preferably, no compression stage is located between absorption stage and concentration stage, and instead the pressure conditions maintained within the extractive distillation zone are those automatically established in the extractive distillation zone in the absence of compression and/or pressure reduction stages within the extractive distillation zone, so that the pressure at the bottom of the extractive distillation zone corresponds at least to the pressure at the top of the extractive distillation zone, in line with the usual pressure loss. As a rule, the pressure difference between top and bottom of the extractive distillation zone is from 0.1 to 3, preferably from 0.2 to 2, bar.
In general, pressures of from 1 to 9, preferably from 2 to 8, in particular from 3 to 7, bar are used in the extractive distillation zone. The pressures in the lower third of the extractive distillation zone, i.e. in the region which is occupied by the lower trays of the extractive distillation zone, which correspond to about a third of the total number of trays of the extractive distillation zone, are as a rule from 1.5 to 9, preferably from 2.5 to 8, in particular from 3.5 to 7, bar.
The extract taken off from the extractive distillation zones is first brought to a higher pressure than the pressure in the extraction distillation zone.
This can be effected, for example, by means of a liquid pump. In general, this pressure increase is effected essentially isothermally, i.e. the only temperature changes which occur, for example a temperature increase up to 1 xc2x0C., are those which are caused by the measure leading to the pressure increase, for example the pumping process. In general, the extract is brought to pressures which are from 1 to 20, preferably from 2 to 18, in particular from 3 to 15, bar above the pressure in the extractive distillation zone, in particular above the pressure in the lower third of the extractive distillation zone.
The extract under increased pressure is then heated in a heat exchange zone by indirect heat exchange with the selective solvent obtained as a bottom product from the solvent recovery zone. The selective solvent is recycled to the extractive distillation zone after the heat exchange. As a result of the heat exchange with the selective solvent, the temperature of the extract is generally increased by from 5 to 80xc2x0 C., preferably 10 to 70xc2x0 C., in particular from 15 to 60xc2x0 C.
The heated extract is then let down by flash evaporation to a pressure which corresponds at least to the pressure in the extractive distillation zone, preferably at least to the pressure in the lower third of the extractive distillation zone, and is higher than the pressure in the downstream solvent recovery zone. It is critical for the pressure reduction that the vapor fraction of the extract, which fraction forms in the flash evaporation, can be returned to the extractive distillation zone without a compression stage. Accordingly, as a rule the pressure is let down in the flash evaporation to pressures which are from 0.05 to 2.0, preferably from 0.1 to 1, bar above the pressure at the feed point of the vapor fraction of the extract into the extractive distillation zone. The flash evaporation is carried out, for example, in an apparatus comprising a pressure reduction valve on an adiabatic evaporator, if required a phase separation vessel being provided downstream for better separation of the vapor and liquid phases forming in the flash evaporation.
The combination of heat exchange zone for the heat exchange between the extract from the extractive distillation zone and the selective solvent recycled from the solvent recovery zone with the downstream flash evaporation can be used in one stage. However, it is also possible to use more than one such combination, for example from 2 to 4, preferably 2 or 3, such combinations, advantageously connected in series. By using more than one of these heat exchange/flash evaporation stages and recycling the part-streams thus obtained to different feed points of the extractive distillation zone, the required separation efficiency of the extractive distillation and the dimensions of the extractive distillation column can be reduced. It is also possible to connect a further heat exchange zone between the last flash evaporation zone and the solvent recovery zone.
That vapor fraction of the extract which forms in the flash evaporation zone or zones and generally comprises from 20 to 80, preferably from 40 to 70, % by weight of the hydrocarbons in the extract is returned to the extractive distillation zone. In general, the returned vapor phase is passed into the lower third of the extractive distillation zone, preferably at the bottom of the extractive distillation zone, for example at a point which is located roughly at the height of the lowermost column tray. In the stepwise flash evaporation, vapor fractions contained in the individual stages can be returned, separately or after their combination, to the extractive distillation zone.
The liquid phase of the extract from the extractive distillation zone, which phase remains after flash evaporation, is fed to a solvent recovery zone which is operated at a lower pressure than the pressure in the flash evaporation zone. The remaining liquid extract phase is let down to the lower pressure in the solvent recovery zone, advantageously by means of an intermediate pressure reduction valve. In general, the pressure in the solvent recovery zone is from 0.1 to 8, preferably from 0.5 to 7, in particular from 1 to 6, bar lower than the pressure in the flash evaporation zone or zones. The solvent recovery zone may be operated, for example, as a gas expulsion zone or as a solvent stripper. In general, heat is supplied to the solvent recovery zone, for example via an indirect heat exchanger using steam (reboiler).
The NMP obtained as a bottom product of the solvent recovery zone and freed from the hydrocarbons is returned to the extraction stages (I) and (III) via the heat exchange zone in which the heat exchange with the extract from the extractive distillation zone takes place.
The product which is obtained from the solvent recovery zone contains the hydrocarbons and is in general taken off as a top stream or as a top and side stream, passes partly or if necessary completely initially through a compression stage and is fed to the extractive distillation zone after the compression. In the compression zone, the hydrocarbon stream is compressed to a pressure which corresponds at least to the pressure in the extractive distillation zone. In general, the hydrocarbon stream is compressed to pressures which are from 0.05 to 2, preferably from 0.1 to 1, bar above the pressure at the feed point of the vapor fraction of the extract into the extractive distillation zone.
FIG. 3 is a schematic diagram of an embodiment of the preferred variant. In this embodiment, 2 extractive distillation zones are connected in series. The first extractive distillation zone is formed by column 1 and the upper tapered column section 2, while the second extractive distillation zone is formed by column 4 and the lower column section 3. The NMP is fed to the upper part of column 1 through line 5 and to the upper part of column 4 through line 6. A C4-hydrocarbon mixture is fed to the bottom of column 1 via line 7.
The columns 2/3 and 4 are also directly connected. A gaseous part stream is removed from the column 2 and washed countercurrently with solvent through line 6.
At the top of the column 1, a refined product which consists essentially of butenes and butanes is taken off through line 8.
At the top of the column 4, an essentially pure 1,3-butadiene is taken off through line 9.
A gas stream containing essentially the other hydrocarbons and other impurities is removed via the side take-off of the column 10 through line 11.
The pressure in the column section 3 is about 5 bar. The extract taken off via line 12 is brought to 15 bar by a liquid pump 13 and then heated at from 70?C [sic] to 125?C [sic] in heat exchanger 14 by means of the NMP taken off from gas expulsion zone 10 via line 24 and essentially free of C4-hydrocarbons. The heated extract is then passed through pressure reduction valve 15 and let down to a pressure slightly above 5 bar. While the gaseous phase formed in the phase separation tank 16 is immediately returned through line 22 and 23 to the column 3, the liquid phase, obtained after the flash evaporation, of the extract is fed through line 17 to another pressure reduction valve 18 where the pressure falls to the level of pressure in the column 10, normally 1.5 bar.
At the top of column 10, a hydrocarbon stream is taken off through line 19 and, after compression in the compressor 20, also fed through line 23 to the bottom of the column 3. It is important, for safety reasons, in this connection that the gas stream 19 is cooled by heat exchange (not depicted in FIG. 3) before entering the compressor 20 so that the temperature of the gas stream after emergence from the compressor does not exceed 110xc2x0 C. The gas stream is normally cooled to 45xc2x0 C.
The NMP which is virtually free of C4-hydrocarbons and is taken off through line 24 and cooled in the heat exchanger 14 is fed through line 25 to the heat exchanger 2. It then passes through another heat exchanger (not depicted in FIG. 3) in which the temperature of the solvent is adjusted to 38xc2x0 C. The amount of solvent arriving through line 26 is then divided into two part streams: line 6 leads to the column 4, while line 27 terminates in the additional extractive distillation zone (III) for separating butenes and butanes. The solvent returns from there through line 5.
In order to simplify the drawings, all the abovementioned back-wash zones with the flows of liquid hydrocarbons back to columns 1 and 4 have also been omitted. The abovementioned side column on the main column 10 is also absent from FIG. 3.